The electrolytes currently used in electrochemical devices such as lithium/lithium ion batteries, hydrogen ion fuel cells, and solar cells are typically liquid or gel electrolytes. However, these liquid or gel electrolytes, although having good room temperature conductivities of >1×10−3 S/cm, have safety concerns such as leakage, explosions due to volatile solvents, dendrite formation, and faster formation/migration of degradation products than in a solid electrolyte (Xu, K., Nonaqueous, 2004, Chemical Reviews 104, (10), 4303-4417; Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H., Solid State Ionics 2002, 148, (3-4), 405-416; Brissot, C.; Rosso, M.; Chazalviel, J. N.; Lascaud, S., J of the Electrochemical Society 1999, 146, (12), 4393-4400). Therefore, new materials with architectures that foster enhanced ion migration over a wide temperature range are needed to replace these flammable liquid or gel electrolytes in electrochemical devices.
Solid state electrolytes have previously been investigated because of the expected increase in safety associated with solid state materials, but these electrolytes typically have relatively poor ionic conductivity (Zaghib et al., 2011, J of Power Sources 196, 3949-3954). Currently available solid electrolytes with the highest ionic conductivities are ceramic/glass and other inorganic superionic conductors, with conductivities potentially in the range of 10−3 to 10−2 S/cm (Fergus, 2010, J of Power Sources 195, 4554-4569). In the case of inorganic superionic conductors, the crystalline systems are typically more conductive than the glasses (Kanno and Maruyama, 2001, Journal of the Electrochemical Society 148 (7), A742-A746). The first reported Li+ ion superionic conductor, Li3N, with a high RT ionic conductivity (6×10−3 S/cm) for a solid electrolyte, has a low electrochemical stability window making it unsuitable as a solid electrolyte (Alpen et al., 1977, Applied Physics Letters 30 (12), 621-62; Lapp et al., 1983, Solid State Ionics 11 (2), 97-103). Other inorganic superionic solid electrolytes such as the crystalline oxide perovskite lithium lanthanum titanates (La0.5Li0.5TiO3) (Inaguma et al., 1993, Solid State Communications 86 (10), 689-693), a series of sulfide crystals such as Li4-xGe1-xPxS4 with the framework structure of γ-Li3PO4, referred to as thio-LISICON (e.g. Li3.25Ge0.25P0.75S4) (Kanno and Maruyama, 2001, Journal of the Electrochemical Society 148 (7), A742-A746), glass ceramics (70Li2S-30P2Ss) (Mizuno et al., 2005, Advanced Materials 17 (7), 918-921; Hayashi et al., 2008, Journal of Materials Science 43 (6), 1885-1889) and glassy materials (Li2S—SiS2—Li3PO4) (Kondo et al., 1992 Solid State Ionics 53, 1183-1186; Takada et al., 1993, Journal of Power Sources 43 (1-3), 135-141), have better electrochemical stability but lower ionic conductivity (˜10−3 S/cm). Only Li2.9PO3.3N0.46 (LiPON) is used commercially as a solid electrolyte in microbatteries (Bates et al., 1992, Solid State Ionics 53, 647-654; Bates et al., 1993, Journal of Power Sources 43 (1-3), 103-110). The highest RT ionic conductivities for lithium superionic conductors have recently been reported for Li10GeP2S12 (12 mS/cm). Substitution of Sn for Ge also forms a superionic crystal, Li10SnP2S12 (7 mS/cm), and both materials are metastable (Bron et al., 2013, J Am Chem Soc 135 (42), 15694-15697; Mo et al., 2012, Chemistry of Materials 24 (1), 15-17). However, these electrolytes are brittle, and they have poor adhesion to electrodes due to changes in volume during successive charge/discharge cycles.
Soft-solid electrolytes exhibit desirable flexibility, but have lower conductivity than ceramic/glass/inorganic conductors (e.g., conductivities in the range of 10−7 to 10−5 S/cm). Examples of soft-solid electrolytes include polyethylene oxide (PEO) (Abitelli et al., 2010, Electrochimica Acta 55, 5478-5484), PEO/composite blends (Croce et al., 1998, Nature 394, 456-458; Croce et al., 1999, J of Physical Chemistry B 103, 10632-10638; Stephan et al., 2009, J of Physical Chemistry B 113, 1963-1971; Zhang et al., 2010, Electrochimica Acta 55, 5966-5974; Zhang et al., 2011, Materials Chemistry and Physics 121, 511-518; Zhan et al., 2011, J of Applied Electrochemistry 40, 1475-1481; Uvarov, 2011, J of Solid State Electrochemistry 15, 367-389), PEO copolymers/blends (Tsuchida et al., 1988, Macromolecules 21, 96-100; Ryu et al., 2005, J of the Electrochemical Society 152, A158-A163; Park et al., 2004, Electrochimica Acta 50, 375-378), molecular or ionic plastic crystals (Timmermans, 1961, J of Physics and Chemistry of Solids 18, 1-8; Sherwood, 1979, The Plastically Crystalline State: Orientationally Disordered Crystals, Wiley, Chichester, UK; MacFarlane and Forsyth, 2001, Advanced Materials 13, 957-966; Pringle et al., 2010, J of Materials Chemistry 20, 2056-2062; Cooper and Angell, 1986, Solid State Ionics 18-9, 570-576; Yoshizawa-Fujita et al., 2007, Electrochemistry Communications 9, 1202-1205.), and low molecular weight glymes (Henderson et al., 2003, Chemistry of Materials 15, 4679-4684; Henderson et al., 2003, Chemistry of Materials 15, 4685-4690; Seneviratne et al., 2004, J of Physical Chemistry B 108, 8124-8128; Andreev et al., 2005, Chemistry of Materials 17, 767-772; Henderson et al., 2005, Chemistry of Materials 17, 2284-2289; Henderson, 2006, J of Physical Chemistry B 110, 13177-13183; Zhang et al., 2007, Angewandte Chemie-International Edition 46, 2848-2850; Zhang et al., 2007, J of the American Chemical Society 129, 8700-8701). Another example of a soft-solid electrolytic material is NAFION™ polymer, which has a hydrophobic perfluorinated matrix that contains anion-coated (typically —SO3−) percolating clusters, and channels through which oppositely charged ions can migrate (Mauritz and Moore, 2004, Chemical Reviews 104, 4535-4586).
For PEO systems, conductivity has been shown to occur primarily through the amorphous phase, where ion migration is coupled to slow backbone segmental motions (Borodin and Smith, 2006, Macromolecules 39, 1620-1629), so that decreases in crystallinity (Abitelli et al., 2010, Electrochimica Acta 55, 5478-5484; Stephan et al., 2009, J of Physical Chemistry B 113, 1963-1971; Zhang et al., 2010, Electrochimica Acta 55, 5966-5974; Zhan et al., 2011, J of Applied Electrochemistry 40, 1475-1481), and alignment of polymer chains (Bruce, 1996, Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences 354, 1577-1593; Andreev and Bruce, 2000, Electrochimica Acta 45, 1417-1423), increase conductivity.
Other approaches to improve ionic conductivities in soft-solid electrolytes are based on the observation that molecular organization rather than disordered structures foster ion mobility. In particular, this is true for materials in which there are alternative, low activation energy pathways for ion migration, such as along and between organized, aligned polymer or liquid crystalline polymer chains (Andreev and Bruce, 2000, Electrochimica Acta 45, 1417-1423; Golodnitsky and Peled, 2000, Electrochimica Acta 45, 1431-1436; Dias et al., 1998, Electrochimica Acta 43, 1217-1224; Hubbard et al., 1998, Electrochimica Acta 43, 1239-1245; Imrie et al., 1999, Advanced Materials 11, 832-834; Yoshio et al., 2004, J of the American Chemical Society 126, 994-995; Kishimoto et al., 2003, J of the American Chemical Society 125, 3196-3197; Yoshio, 2006, J of the American Chemical Society 128, 5570-5577; Shimura et al., 2008, J of the American Chemical Society 130, 1759-1765; Ichikawa, 2011, J of the American Chemical Society 133, 2163-2169); along polymeric/inorganic nanoparticle interfaces, possibly due to weakening of the ether O—Li+ bond (Shen, 2009, Electrochimica Acta 54, 3490-3494; Chen-Yang et al., 2008, J of Power Sources 182, 340-348; Marcinek et al., 2000, J of Physical Chemistry B 104, 11088-11093; Borodin et al., 2003, Macromolecules 36, 7873-7883); and along ion channels in low molecular weight glymes and trilithium compounds (Gadjourova et al., 2001, Nature 412, 520-523; MacGlashan et al., 1999, Nature 398, 792-794; Gadjourova et al., 2001, Chemistry of Materials 13, 1282-1285; Stoeva et al., 2003, J of the American Chemical Society 125, 4619-4626; Staunton et al., 2005, J of the American Chemical Society 127, 12176-12177; Zhang et al., 2007, J of the American Chemical Society 129, 8700-8701; Zhang et al., 2008, Chemistry of Materials 20, 4039-4044; Moriya et al., 2012, Chemistry-A European J 18, 15305-15309). Decreased interactions between mobile cations such as Li30  and their associated anions and/or solvating matrix, such as in microphase separated solid polymer electrolytes (SPEs) have also been shown to increase cation mobility and conductivity (Ryu et al., 2005, J of the Electrochemical Society 152, A158-A163). For the design of soft solid electrolytes with higher conductivities, crystalline solids in which channel walls have low affinity for the enclosed ions are desired.
Key problems that remain for the use of solid electrolytes in all solid-state Li batteries, aside from the general concerns of stability windows and compatibility with solvents when used in air or liquid flow-through cathodes, are improvements in room temperature ionic conductivities, increased charge/discharge rates, high lithium ion transference numbers to avoid polarization effects, and the maintenance of good electrode/electrolyte contact during the volume changes that occur in the electrodes during repeated charge/discharge cycles (Doyle et al., 1994, Electrochimica Acta 39, (13), 2073-81; Thomas et al., 2000, J of Power Sources 89, (2), 132-138; Gadjourova et al., 2001, Nature 412, (6846), 520-523). The engineering of solid-state organic materials with specific ion conduction pathways that can enhance ion migration offers promise as a means to achieve higher solid-state ionic conductivities, while soft, more malleable organics may better adhere to electrodes. However, there has been only limited progress in this area.
In addition, materials research for sodium ion batteries, which is highly desirable because sodium is more abundant than lithium, has focused on the cathodes (Lu et al., 2010, J of Power Sources 195, (9), 2431-2442; Lu et al., 2010, JOM 62, (9), 31-36; Kim et al., 2012, Advanced Energy Materials 2, (7), 710-721; Ellis and Nazar, 2012, Current Opinion in Solid State & Materials Science 16, (4), 168-177; Palomares et al., 2012, Energy & Environmental Science 2012, 5, (3), 5884-5901). There has been less research on the electrolytes for sodium ion batteries, for which the sodium salts are less soluble than lithium salts in aprotic solvents. Inorganic materials such as NASICON Na+ superionic conductors are the most common solid sodium electrolytes investigated (Fuentes et al., 2001, Solid State Ionics 140, (1-2), 173-179). However, there have also been studies on polyethylene oxide (Ma et al., 1993, J of the Electrochemical Society 140, (10), 2726-2733; Mohan et al., 2007, Soft Materials 5, (1), 33-46; Mohan et al., 2007, J of Polymer Research 14, (4), 283-290; Kumar et al., 2011, Physica B-Condensed Matter 406, (9), 1706-1712) or polyvinyl alcohol-based polymer electrolytes (Bhargav et al., 2007, Ionics 13, (3), 173-178; Bhargav et al., 2007, Ionics 2007, 13, (6), 441-446), as well as polymer gels (Kumar and Hashmi, S. A., 2010, Solid State Ionics 181, (8-10), 416-423; Kumar et al., 2011, Solid State Ionics 202, (1), 45-53; Kumar et al., 2011, Solid State Ionics 201, (1), 73-80). The same considerations and requirements for Na-ion solid electrolytes are necessary as for Li-ion solid electrolytes.
Thus, there is a continuing need in the art for solid state electrolytes, including solid sodium electrolytes, for electrochemical devices. The present invention addresses this continuing need in the art.